This invention relates to a multi-stage combustion method capable of effectively inhibiting the formation of nitrogen oxides.
It has been desired to provide combustion methods capable of effectively inhibiting the formation of nitrogen oxides (NO.sub.x) which produce photochemical oxidants.
The nitrogen oxides formed in combustion furnaces include: (a) nitrogen monoxide (hereinafter referred to as "fuel NO") resulting from the oxidation of nitrogen components contained in various fuels, (b) nitrogen monoxide (hereinafter referred to as "prompt NO") promptly formed when hydrocarbon fuels such as fuel oil, kerosene and LPG are burned at an air ratio (the ratio of the actual air supply to the amount of air stoichiometrically required for the combustion of fuel) of about 0.5 to 1.4, permitting hydrocarbons to react with the nitrogen in the air and further to undergo several reactions, and (c) nitrogen monoxide (hereinafter referred to as "thermal NO") produced when the nitrogen and oxygen in the air react at a high temperature in the course of combustion.
Main combustion methods heretofore known for inhibiting nitrogen oxides are:
(1) A method in which air is supplied in two stages to form a first-stage combustion zone having an air ratio of up to 1.0 and a second-stage combustion zone downstream from the first-stage zone with a supplemental air supply.
(2) A method which uses a combustion furnace equipped with a plurality of burners and in which air is supplied to each burner at an excessive or somewhat insufficient rate relative to the fuel supply to effect combustion in a nonequivalent mode.
(3) A method in which the exhaust gas resulting from combustion is admixed with the fuel on the air for combustion by circulation.
The method (1) is unable to suppress the formation of prompt NO when the air ratio of the first-stage combustion zone is in the usual range of 0.5 to 1.0. Even if it is attempted to inhibit the formation of prompt NO to the greatest possible extent by maintaining the air ratio at about 0.5, the unburned components will react with the secondary air where it is supplied, giving prompt NO. Thus the method fails to produce the desired result. With the method (2) in which the fuel is burned at an air ratio (usually 0.6 to 1.4) at which each burner can burn the fuel independently of another, the formation of thermal NO and prompt NO inevitably results. The method (3) is not fully feasible since the exhaust gas, if circulated at an increased rate to effectively inhibit NO.sub.x, will impair steady combustion.